/*
** 2018-02-24
**
** The author disclaims copyright to this source code.  In place of
** a legal notice, here is a blessing:
**
**    May you do good and not evil.
**    May you find forgiveness for yourself and forgive others.
**    May you share freely, never taking more than you give.
**
************************************************************************
**
** This file contains an adjusted version of function sqlite3RunVacuum
** to allow reducing or removing reserved page space.
** For this purpose the number of reserved bytes per page for the target
** database is passed as a parameter to the adjusted function.
**
** NOTE: When upgrading to a new version of SQLite3 it is strongly
** recommended to check the original function sqlite3RunVacuum of the
** new version for relevant changes, and to incorporate them in the
** adjusted function below.
**
** The code below is based on SQLite version 3.24.0 - check when updating SQLite.
*/

/*
** This routine implements the OP_Vacuum opcode of the VDBE.
*/
/* CHANGE 1 of 3: Add function parameter nRes */
SQLITE_PRIVATE int sqlite3RunVacuumForRekey(char **pzErrMsg, sqlite3 *db, int iDb, int nRes){
  int rc = SQLITE_OK;     /* Return code from service routines */
  Btree *pMain;           /* The database being vacuumed */
  Btree *pTemp;           /* The temporary database we vacuum into */
  u16 saved_mDbFlags;     /* Saved value of db->mDbFlags */
  u32 saved_flags;        /* Saved value of db->flags */
  int saved_nChange;      /* Saved value of db->nChange */
  int saved_nTotalChange; /* Saved value of db->nTotalChange */
  u8 saved_mTrace;        /* Saved trace settings */
  Db *pDb = 0;            /* Database to detach at end of vacuum */
  int isMemDb;            /* True if vacuuming a :memory: database */
  /* CHANGE 2 of 3: Do not define local variable nRes */
  /*int nRes;*/               /* Bytes of reserved space at the end of each page */
  int nDb;                /* Number of attached databases */
  const char *zDbMain;    /* Schema name of database to vacuum */

  if (!db->autoCommit) {
    sqlite3SetString(pzErrMsg, db, "cannot VACUUM from within a transaction");
    return SQLITE_ERROR;
  }
  if (db->nVdbeActive>1) {
    sqlite3SetString(pzErrMsg, db, "cannot VACUUM - SQL statements in progress");
    return SQLITE_ERROR;
  }

  /* Save the current value of the database flags so that it can be
  ** restored before returning. Then set the writable-schema flag, and
  ** disable CHECK and foreign key constraints.  */
  saved_flags = db->flags;
  saved_mDbFlags = db->mDbFlags;
  saved_nChange = db->nChange;
  saved_nTotalChange = db->nTotalChange;
  saved_mTrace = db->mTrace;
  db->flags |= SQLITE_WriteSchema | SQLITE_IgnoreChecks;
  db->mDbFlags |= DBFLAG_PreferBuiltin | DBFLAG_Vacuum;
  db->flags &= ~(SQLITE_ForeignKeys | SQLITE_ReverseOrder | SQLITE_CountRows);
  db->mTrace = 0;

  zDbMain = db->aDb[iDb].zDbSName;
  pMain = db->aDb[iDb].pBt;
  isMemDb = sqlite3PagerIsMemdb(sqlite3BtreePager(pMain));

  /* Attach the temporary database as 'vacuum_db'. The synchronous pragma
  ** can be set to 'off' for this file, as it is not recovered if a crash
  ** occurs anyway. The integrity of the database is maintained by a
  ** (possibly synchronous) transaction opened on the main database before
  ** sqlite3BtreeCopyFile() is called.
  **
  ** An optimisation would be to use a non-journaled pager.
  ** (Later:) I tried setting "PRAGMA vacuum_db.journal_mode=OFF" but
  ** that actually made the VACUUM run slower.  Very little journalling
  ** actually occurs when doing a vacuum since the vacuum_db is initially
  ** empty.  Only the journal header is written.  Apparently it takes more
  ** time to parse and run the PRAGMA to turn journalling off than it does
  ** to write the journal header file.
  */
  nDb = db->nDb;
  rc = execSql(db, pzErrMsg, "ATTACH''AS vacuum_db");
  if (rc != SQLITE_OK) goto end_of_vacuum;
  assert((db->nDb - 1) == nDb);
  pDb = &db->aDb[nDb];
  assert(strcmp(pDb->zDbSName, "vacuum_db") == 0);
  pTemp = pDb->pBt;

  /* The call to execSql() to attach the temp database has left the file
  ** locked (as there was more than one active statement when the transaction
  ** to read the schema was concluded. Unlock it here so that this doesn't
  ** cause problems for the call to BtreeSetPageSize() below.  */
  sqlite3BtreeCommit(pTemp);

  /* CHANGE 3 of 3: Do not call sqlite3BtreeGetOptimalReserve */
  /* nRes = sqlite3BtreeGetOptimalReserve(pMain); */

  /* A VACUUM cannot change the pagesize of an encrypted database. */
#ifdef SQLITE_HAS_CODEC
  if (db->nextPagesize) {
    extern void sqlite3CodecGetKey(sqlite3*, int, void**, int*);
    int nKey;
    char *zKey;
    sqlite3CodecGetKey(db, iDb, (void**)&zKey, &nKey);
    if (nKey) db->nextPagesize = 0;
  }
#endif

  sqlite3BtreeSetCacheSize(pTemp, db->aDb[iDb].pSchema->cache_size);
  sqlite3BtreeSetSpillSize(pTemp, sqlite3BtreeSetSpillSize(pMain, 0));
  sqlite3BtreeSetPagerFlags(pTemp, PAGER_SYNCHRONOUS_OFF | PAGER_CACHESPILL);

  /* Begin a transaction and take an exclusive lock on the main database
  ** file. This is done before the sqlite3BtreeGetPageSize(pMain) call below,
  ** to ensure that we do not try to change the page-size on a WAL database.
  */
  rc = execSql(db, pzErrMsg, "BEGIN");
  if (rc != SQLITE_OK) goto end_of_vacuum;
  rc = sqlite3BtreeBeginTrans(pMain, 2);
  if (rc != SQLITE_OK) goto end_of_vacuum;

  /* Do not attempt to change the page size for a WAL database */
  if (sqlite3PagerGetJournalMode(sqlite3BtreePager(pMain))
    == PAGER_JOURNALMODE_WAL) {
    db->nextPagesize = 0;
  }

  if (sqlite3BtreeSetPageSize(pTemp, sqlite3BtreeGetPageSize(pMain), nRes, 0)
    || (!isMemDb && sqlite3BtreeSetPageSize(pTemp, db->nextPagesize, nRes, 0))
    || NEVER(db->mallocFailed)
    ) {
    rc = SQLITE_NOMEM_BKPT;
    goto end_of_vacuum;
  }

#ifndef SQLITE_OMIT_AUTOVACUUM
  sqlite3BtreeSetAutoVacuum(pTemp, db->nextAutovac >= 0 ? db->nextAutovac :
    sqlite3BtreeGetAutoVacuum(pMain));
#endif

  /* Query the schema of the main database. Create a mirror schema
  ** in the temporary database.
  */
  db->init.iDb = nDb; /* force new CREATE statements into vacuum_db */
  rc = execSqlF(db, pzErrMsg,
    "SELECT sql FROM \"%w\".sqlite_master"
    " WHERE type='table'AND name<>'sqlite_sequence'"
    " AND coalesce(rootpage,1)>0",
    zDbMain
  );
  if (rc != SQLITE_OK) goto end_of_vacuum;
  rc = execSqlF(db, pzErrMsg,
    "SELECT sql FROM \"%w\".sqlite_master"
    " WHERE type='index'",
    zDbMain
  );
  if (rc != SQLITE_OK) goto end_of_vacuum;
  db->init.iDb = 0;

  /* Loop through the tables in the main database. For each, do
  ** an "INSERT INTO vacuum_db.xxx SELECT * FROM main.xxx;" to copy
  ** the contents to the temporary database.
  */
  rc = execSqlF(db, pzErrMsg,
    "SELECT'INSERT INTO vacuum_db.'||quote(name)"
    "||' SELECT*FROM\"%w\".'||quote(name)"
    "FROM vacuum_db.sqlite_master "
    "WHERE type='table'AND coalesce(rootpage,1)>0",
    zDbMain
  );
  assert((db->mDbFlags & DBFLAG_Vacuum) != 0);
  db->mDbFlags &= ~DBFLAG_Vacuum;
  if (rc != SQLITE_OK) goto end_of_vacuum;

  /* Copy the triggers, views, and virtual tables from the main database
  ** over to the temporary database.  None of these objects has any
  ** associated storage, so all we have to do is copy their entries
  ** from the SQLITE_MASTER table.
  */
  rc = execSqlF(db, pzErrMsg,
    "INSERT INTO vacuum_db.sqlite_master"
    " SELECT*FROM \"%w\".sqlite_master"
    " WHERE type IN('view','trigger')"
    " OR(type='table'AND rootpage=0)",
    zDbMain
  );
  if (rc) goto end_of_vacuum;

  /* At this point, there is a write transaction open on both the
  ** vacuum database and the main database. Assuming no error occurs,
  ** both transactions are closed by this block - the main database
  ** transaction by sqlite3BtreeCopyFile() and the other by an explicit
  ** call to sqlite3BtreeCommit().
  */
  {
    u32 meta;
    int i;

    /* This array determines which meta meta values are preserved in the
    ** vacuum.  Even entries are the meta value number and odd entries
    ** are an increment to apply to the meta value after the vacuum.
    ** The increment is used to increase the schema cookie so that other
    ** connections to the same database will know to reread the schema.
    */
    static const unsigned char aCopy[] = {
      BTREE_SCHEMA_VERSION,     1,  /* Add one to the old schema cookie */
      BTREE_DEFAULT_CACHE_SIZE, 0,  /* Preserve the default page cache size */
      BTREE_TEXT_ENCODING,      0,  /* Preserve the text encoding */
      BTREE_USER_VERSION,       0,  /* Preserve the user version */
      BTREE_APPLICATION_ID,     0,  /* Preserve the application id */
    };

    assert(1 == sqlite3BtreeIsInTrans(pTemp));
    assert(1 == sqlite3BtreeIsInTrans(pMain));

    /* Copy Btree meta values */
    for (i = 0; i<ArraySize(aCopy); i += 2) {
      /* GetMeta() and UpdateMeta() cannot fail in this context because
      ** we already have page 1 loaded into cache and marked dirty. */
      sqlite3BtreeGetMeta(pMain, aCopy[i], &meta);
      rc = sqlite3BtreeUpdateMeta(pTemp, aCopy[i], meta + aCopy[i + 1]);
      if (NEVER(rc != SQLITE_OK)) goto end_of_vacuum;
    }

    rc = sqlite3BtreeCopyFile(pMain, pTemp);
    if (rc != SQLITE_OK) goto end_of_vacuum;
    rc = sqlite3BtreeCommit(pTemp);
    if (rc != SQLITE_OK) goto end_of_vacuum;
#ifndef SQLITE_OMIT_AUTOVACUUM
    sqlite3BtreeSetAutoVacuum(pMain, sqlite3BtreeGetAutoVacuum(pTemp));
#endif
  }

  assert(rc == SQLITE_OK);
  rc = sqlite3BtreeSetPageSize(pMain, sqlite3BtreeGetPageSize(pTemp), nRes, 1);

end_of_vacuum:
  /* Restore the original value of db->flags */
  db->init.iDb = 0;
  db->mDbFlags = saved_mDbFlags;
  db->flags = saved_flags;
  db->nChange = saved_nChange;
  db->nTotalChange = saved_nTotalChange;
  db->mTrace = saved_mTrace;
  sqlite3BtreeSetPageSize(pMain, -1, -1, 1);

  /* Currently there is an SQL level transaction open on the vacuum
  ** database. No locks are held on any other files (since the main file
  ** was committed at the btree level). So it safe to end the transaction
  ** by manually setting the autoCommit flag to true and detaching the
  ** vacuum database. The vacuum_db journal file is deleted when the pager
  ** is closed by the DETACH.
  */
  db->autoCommit = 1;

  if (pDb) {
    sqlite3BtreeClose(pDb->pBt);
    pDb->pBt = 0;
    pDb->pSchema = 0;
  }

  /* This both clears the schemas and reduces the size of the db->aDb[]
  ** array. */
  sqlite3ResetAllSchemasOfConnection(db);

  return rc;
}
